The Pollination Story, Part 5; beyond birds - other vertebrate helpers

This is part of an ongoing series on pollination, which I truly believe is one of the world's great stories. Last time we got as far as looking at the role of birds, dream customers for a plant wanting pollen to be taken to distant flowers of the same species by highly mobile animals able to remember subtle cues to make sure they deliver to the right species.

It's only surprisingly recently that the role of other vertebrates, notably mammals, has been likewise recognised. Back in the 1970s research began into the possible role of mammals as pollinators in both South Africa and Australia, but while the South Africans showed that a range of small mammals, notably rodents, small primates and elephant shrews, were effective pollinators, the Australian research petered out for a while.

Cape Rock Elephant-shrew (or Sengi) Elephantulus edwardii with Cytinus visseri.Photo courtesy BBC.

More recently however, long-term studies by the University of Wollongong on the south coast of New South Wales dramatically changed our perception of the importance of mammals as pollinators here. They put traps by flowers of Waratahs Teleopea speciosissima and various Banksia species, all large showy-flowering members of the family Proteaceae, hitherto assumed to be solely bird pollinated.

Sydney Waratah (above) and Heath-leafed Banksia B. ericifolia;two of the seven flower species studied in coastal southern New South Wales.

Mammals were trapped by all seven species, and all of them were carrying pollen in their fur and their droppings. The most abundant species was the supposedly carnivorous Brown Antechinus A. stuartii, followed by Sugar Gliders Petaurus breviceps, Eastern Pygmy Possums Cercartetus nanus and Bush Rats Rattus fuscipes.

Bush Rat, a perhaps unexpected pollinator.

Sugar Gliders, probably less of a surprise.

A further surprise was the observation that they were feeding thoroughly and non-destructively; these were very effective and obviously non-incidental pollinators. The commonest pollinating honeyeater in the area was the Eastern Spinebill Acanthorhynchus tenuirostris - the one most of us would probably have nominated as the most important pollinator operating there. In fact on average the spinebills were carrying around 40 pollen grains each, while the antechinuses were bearing over 400 grains and the Sugar Gliders more than 2000! The gliders were carrying pollen up to 60 metres from the flower, while antechinuses were travelling about half that. (Determined, incidentally, by attaching little spools of thread to their backs and following them the next day.)

The real clincher however came with the application of exclusion experiments - covering flowers with bags by day or night, since all the mammal pollinators were nocturnal, and all the birds (and nearly all the insects) were diurnal. Mammal-only pollinated flowers had three times the seed set of flowers only visited by day.

We now now know of at least 25 Australian marsupials and two rodents which visit flowers; the two rats, six little possums and the strange Western Australian Dibbler Parantechinus apicalis, a carnivore, do so regularly. Of these the best-known is probably the remarkable Honey Possum, or Noolbenger, Tarsipes rostratus, the only mammal in the world known to be entirely dependent on nectar and pollen.

Honey Possums on Eucalyptus macrocarpa.
(Photo from web.)

In general mammal-pollinated flowers will have characteristics including the following. They are likely to have tight inflorescences with strong stems and a high nectar flow - all these characteristics are shared with bird-pollinated flowers. Other characters however, to deter birds, include nocturnal flowering, dull colours, flowers close to the ground or hidden inside foliage. Some are reported to have musky 'mouse-like' scents.

Creeping Banksia Banksia repens, Stirling Ranges NP, Western Australia.
One of several WA mammal-specialising banksias whose dull-coloured flowers grow from
stems on or just under the ground.

In addition to South Africa, studies have also been carried out in Madasgascar and South and Central America, with equivalent results. In Madagascar the key mammal pollinators unsurprisingly were lemurs (both large ones and mouse lemurs); in the Neotropics small monkeys were significant, along with marsupials, rodents and climbing carnivores.

Golden-mantled Tamarin Saguinus tripartitus, Napo Lodge, Yasuní NP, Ecuador.
The tamarins are particularly significant Neotropical pollinators.

It is highly likely that many mammal pollinators in Asian forests remain to be identified, but already there are over 60 pollinating non-flying mammal species known - and this is without considering the most significant furry pollen-transferers.

Grey-headed Fruit Bats Pteropus poliocephalus, Canberra (above)
and Bellingen, northern New South Wales (below).
Scores of bat species feed on blossoms, as well as fruit, and carry the pollen (and seeds)
for tens of kilometres.

In the tropics too, many species of micro-chiropterans (the essentially insectivorous echo-locating small bats) have secondarily also adopted flower-feeding habits, and have become highly specialised pollinators. A lot more work seems to have been done in the Neotropics than elsewhere, so doubtless we have much to learn. Probably thousands of tropical plants rely heavily or even solely on bat pollinators. Many have strong complex scents, including some described as 'plasticky'. Unlike the big fruit bats which visit flowers by climbing nimbly among branches, the little bats simply hover briefly in front of flower after flower. 

One bat-flower story however has grabbed my attention since I first read about it a decade ago. It concerns a tiny Central American blossom bat called Glossophaga commissarisi and a night-flowering vine called Mucuna holtonii.The vine hangs its flowers in clearings and attracts distant bats with a strong sourish-garlicky smell. When the bat gets closer however the bat starts to 'ping', as it would if navigating in tight spaces or hunting insects. In this case though it is examining the flowers. When the flower is ready for pollination, when it also provides nectar, it erects a strange concave petal which perfectly reflects the bat's sonar back to it - the bat can hear that the flower is worth visiting! It briefly grasps the front of the flower and inserts its head, where its long tongue probes the nectar. This act however also sets off a trigger, which causes anthers to emerge and dust the bat's back with pollen... Amazed? I am!

Mucuna holtonii flowers.
Courtesy of La Selva Florula Digital.
There are some amazing pictures on the web of the bat actually pollinating the flower,
but understandably the owners are pretty protective of them, so I can't show you here.
They're worth looking for though!

More recent ingenious experiments with artificial bat heads, emitting and collecting ultrasonic sound directed at a range of tropical flowers, have indicated that each flower tested could at least theoretically be distinguished by a flying bat.

But astonishingly it's not just birds and mammals among vertebrates which pollinate - reptiles, notably lizards, are involved too! This is apparently especially true on islands, where presumably the plants have less choice. In New Zealand and Mauritius geckoes are known pollinators, visiting the flowers for energy in the form of nectar. 

Gecko Phelsuma cepediana on Roussea simplex, Mauritius.
(The plant incidentally is now threatened following the introduction of exotic ants, which
feed on the flowers and repel the geckos.)
Photo courtesy National Geographic.

I'll end though with another Australian story, which I also find almost incredible, of a lizard which doesn't actually pollinate but is essential for the event to take place. On Mount Wellington above Hobart in Tasmania life is tough, and the Honey Bush Richea scoparia protects its reproductive parts in a capsule of fused petals.

Typical Mount Wellington habitat in the foreground, above Hobart (above).
Richea scoparia below.

So how do pollinating insects enter? The answer is - a lizard!

Snow Skink Niveoscincus microlepidus.Courtesy Australian Reptile Online Database.

Snow Skinks are common insect hunters on Mount Wellington (not when I was there last - too cold for any sensible animal to be above ground level!). However in summer they switch almost exclusively to Honey Bush nectar, which they obtain by ripping open the capsules, allowing insects to access anthers and styles. The flowers must mature at the mildest time of the year (on Mt Wellington that is relative!), and advertise the fact to the lizards by turning brown. How I love nature's remarkable stories.

More on pollination in coming weeks; I do hope you're as captivated by the nuances of the story as I am.

MEANTIME, BACK ON TUESDAY